1. Field of the Invention
The present invention relates to an atomic force microscope capable of imaging surfaces in fluids at pressures greater than normal atmospheric pressure. More particularly, the surfaces can be imaged in gas or liquid at temperatures greater than 100xc2x0 C.
2. Description of Related Art
Scanning probe microscopy, particularly atomic force microscopy (AFM), has become an indispensable tool for imaging solid surfaces at resolutions ranging from the atomic scale (for periodic and defect structures) to the microtopographic scale (for roughness, particle analysis, step-terrace patterns, magnetic patterns, microorganisms and biomolecules). AFM is also important for crystal growth studies because it allows not only ex-situ characterization of the spacing and shape of atomic scale steps and terraces, but also in-situ, real-time imaging of step motion and surface kinematics during crystal dissolution or growth in aqueous or other solutions.
A limitation of AFM has been the relatively narrow range of temperatures accessible for in-situ imaging in liquids. The range of crystal dissolution or growth rates accessible to in-situ AFM imaging of step motion is about 10xe2x88x926 to 10xe2x88x9210 moles mxe2x88x922sxe2x88x921. Rates of most oxides and silicates (which are of interest in chemical weathering of rocks, buildings and monuments, radioactive waste storage, industrial pipe scaling, and enhanced oil recovery techniques such as steam-flooding, and other applications) are below this range at room temperature. To apply AFM to the aqueous dissolution and growth of these materials, higher temperatures are needed to hasten the reaction rates. For example, the minimum dissolution rates (dependent on pH) of quartz (crystalline SiO2) and albite (NaAlSi3O8) are higher than 10xe2x88x9210 moles mxe2x88x922sxe2x88x921 at 150xc2x0 C.
Heating stages for ambient AFM have been built (see Musevic et al., Rev. Sci. Instrum. 67, 2554-2556 (1996); Prilliman et al., Rev. Sci. Inst., 69, 3245-3250 (1998) (at http://nanonet.rice.edu/papers/Rev-Sci-Inst-TM-AFM-Heating/)). However, the vapor pressure of the liquid phase imposes a fundamental limitation on fluid cell temperature; a 100xc2x0 C. aqueous solution will boil if the pressure is not greater than the vapor pressure of water at temperature. Moreover, practical temperature limits can be significantly lower than the boiling point of the solution. Exsolution (e.g., of dissolved CO2 and O2) and bubble formation interferes with imaging unless the source solution is degassed upstream of the AFM sample cell; this is generally accomplished by overheating at the source, which means, if pressurization is not possible, that the sample cell temperature must be lower than the ambient boiling point. In addition, some experiments (such as dissolution or growth of carbonates) requires use of dissolved gases, so that bubble formation rather than boiling imposes a temperature limit.
A need exists for an AFM that allows observation of atomic scale phenomena in liquids or gases at temperatures and pressures not currently attainable. The present invention is a design of an AFM capable of imaging in aqueous solution or other fluids at temperatures greater than 100xc2x0 C. and at pressures greater than normal atmospheric pressure.
The object of the present invention is to provide an atomic force microscope (AFM) that can be used to image solid surfaces in fluids, either liquid or gas, at pressures greater than normal atmospheric pressure. Surfaces can be imaged in fluids at temperatures greater than 100xc2x0 C. and greater than 1 atmosphere, with less than 1 nanometer vertical resolution.
Internal pressurization of the microscope is achieved in two separate chambers: the base chamber and the sample cell. The AFM has a gas pressurized microscope base chamber, which houses the stepper motor for coarse advance and the piezoelectric scanner. A chemically inert, flexible membrane separates this base chamber from the sample cell and constrains a high temperature, pressurized liquid or gas in the sample cell, while allowing three-dimensional motion of the sample by means of the piezoelectric scanner element. The membrane prevents any fluid from leaking from the sample cell into the gas pressurized base chamber. All electromechanical and mechanical components are reliably separated from any liquid in the sample cell.
The sample cell has inlet and outlet ports for the continuous flow of gas or liquid through the sample environment; other ports can be added for various probes, such as a temperature transducer or pH monitor. Fluid flow through the sample cell is controlled by means of a back-pressure regulator or mass flow controller. An optically transparent window on the sample cell allows an AFM laser optical head to be used for detection of cantilever deflections inside the sample cell. Resistive heating is used to maintain isothermal conditions in the sample cell. In an alternative embodiment, a second fluid cell and membrane are situated between the sample cell and the base chamber. The second cell can be used to prevent bubble formation in the sample cell caused by gas permeating from the gas pressurized base chamber through the membrane into the sample chamber.
The present invention overcomes current limitations on the temperature and pressure range accessible to AFM imaging, particularly in aqueous solutions under hydrothermal conditions. Immediate applications of this AFM include the study of surface chemical and redox reactions at nanometer scales and at temperatures sufficient to increase the net reaction rates for many materials so as to be observable by atomic force microscopy. This invention is of interest in the fields of geochemistry, environmental science, materials science such as semiconductor device manufacturing, and other areas of manufacturing that use processes occurring at the nanometer scale. Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings.